window.onscroll = function(){
var back = document.getElementById("back");

    var scroll_top = document.body.scrollTop || document.documentElement.scrollTop;
    if( scroll_top > 180){
        back.style.display = "block"; 
    }else{
        back.style.display = "none";
    } 
};

var back2 = document.getElementsByClassName("shang")
console.log(back2);
back2.onclick = function(){
    console.log(888);
    scrollTo({
        top:0,
        behavior:"smooth",
    })
}